Hollow valve for internal combustion engine, and internal combustion engine having the hollow valve

ABSTRACT

In a hollow valve for an internal combustion engine, including a stem portion, an umbrella portion provided at one end of the stem portion, a gastight cavity that communicates the interior of the stem portion with that of the umbrella portion, and a coolant contained in the cavity, the umbrella portion has a wall portion (a lower cap) having a combustion-chamber-side wall that forms a part of a wall of a combustion chamber of the engine, and a cavity-side wall that forms a part of a wall of the cavity, and at least one high-thermal-conductivity member is provided on the cavity-side wall, for conduction of heat between the wall portion and one of at least the coolant and the stem portion as a heat-conduction medium.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2007-178867 filed on Jul. 6, 2007, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a hollow valve for an internal combustion engine for use in an intake/exhaust valve actuating mechanism of the internal combustion engine, and also relates to an internal combustion engine in which the hollow valve or valves is/are mounted.

2. Description of the Related Art

In an intake/exhaust valve actuating mechanism of an internal combustion engine, a valve is provided for permitting or interrupting communication between an intake port and the combustion chamber or between an exhaust port and the combustion chamber. Generally, the valve is roughly divided into an umbrella portion and a stem portion, and the stem portion is adapted to reciprocate in its axial direction, so as to create a condition where the intake port and the combustion chamber, or the exhaust port and the combustion chamber, communicate with each other, or a condition where communication between the intake port or exhaust port and the combustion chamber is interrupted.

Here, the umbrella portion of the valve, in particular, a portion of the umbrella portion that forms a part of the wall of the combustion chamber, is exposed to combustion gas present in the combustion chamber, and therefore, heat generated through combustion is likely to be transferred to that portion of the umbrella portion. Especially in recent years, the amount of heat generated in the combustion chamber has a tendency of increasing, due to its combustion modes for reducing the fuel consumption and pollutants (such as hydrocarbon) contained in exhaust gas, while achieving high power or output of the engine. As a result, the heat transferred to the umbrella portion is increased.

Under the circumstances as described above, a hollow valve for an internal combustion engine is disclosed in, for example, Japanese Patent Application Publication No. 5-113113 (JP-A-5-113113), in which a gastight cavity that extends from an umbrella portion to a stem portion is formed, and a green compact serving as a heat transfer medium is contained in the cavity, so that heat transferred to the umbrella portion is likely to be dissipated. In the hollow engine valve in which the cavity is not filled with the heat transfer medium, the heat transfer medium is shaken in the cavity under the reciprocating motion of the valve, to flow between the umbrella portion and the stem portion. In this manner, the heat transfer medium can transfer the heat of the umbrella portion received from the combustion chamber, to the stem portion, for dissipation of the heat, thereby to suppress or avoid an excessive temperature rise in the umbrella portion.

In the meantime, Japanese Patent Application Publication No. 2003-188324 is concerned with a heat sinking or radiating substrate of a semiconductor device, which is not related to the hollow valve for the internal combustion engine. To provide the heat sinking substrate disclosed in this publication, rods (formed of at least one of a carbon-fiber reinforced composite material, a carbon-based metal composite material, a high-thermal-conductivity metallic material, etc.) having a higher thermal conductivity than a base material are embedded in the base material in the vertical direction, so as to achieve good heat transfer characteristics.

Although a wide variety of types of internal combustion engines are available, the axial directions of hollow valves for engines do not coincide with the direction of gravity (i.e., the vertical direction) in most of the types of the engines. Therefore, the heat transfer medium (coolant) does not necessarily contact uniformly with the respective walls of the umbrella portion and stem portion in the cavity, and the umbrella portion and the stem portion may not be cooled uniformly. For example, a so-called in-line engine is normally inclined and installed on the vehicle, and thus the axial directions of the hollow engine valves are less likely or unlikely to coincide with the direction of gravity. Also, a so-called V-type engine (including a horizontal opposed engine) has banks forming a certain bank angle, and thus the axial directions of the hollow engine valves are less likely or unlikely to coincide with the direction of gravity. Furthermore, since the hollow engine valves are generally inclined and installed on the engines, the axial directions of the valves do not coincide with the direction of gravity.

Namely, in the hollow engine valve of the related art, even if heat is transferred to the heat transfer medium in a certain portion of the umbrella portion in the cavity, so that the heat can be dissipated to an object, such as a stem portion, to which heat is to be transferred, the heat transfer medium may not be able to contact a wall of the cavity in another portion of the umbrella portion, and thus cannot take heat from this portion. As a result, the cooling effect may vary from portion to portion in the umbrella portion for which cooling is particularly needed. While the amount of the heat transfer medium (generally, metallic sodium) may be increased so as to solve this problem, most of the heat transfer medium moves in a direction opposite to that of movements of the valve, and the inertial force of the valve may increase as the amount of the heat transfer medium contained increases. The thus increased inertial force of the valve may become an impediment to smooth reciprocating movements of the hollow engine valve.

SUMMARY OF THE INVENTION

The present invention provides a hollow valve for an internal combustion engine, which has a good cooling capability, and an internal combustion engine in which such hollow valves are mounted.

A first aspect of the invention relates to a hollow valve for an internal combustion engine, which includes: a stem portion, an umbrella portion provided at one end of the stem portion, the stem portion and the umbrella portion cooperating with each other to form a gastight cavity that communicates an interior of the stem portion with an interior of the umbrella portion, and a coolant contained in the cavity. In the hollow engine valve, the umbrella portion has a wall portion having a combustion-chamber-side wall that forms a part of a wall of a combustion chamber of the internal combustion engine, and a cavity-side wall that forms a part of a wall of the cavity, and at least one high-thermal-conductivity member is provided on the cavity-side wall, for conduction of heat between the wall portion of the umbrella portion and one of at least the coolant and the stem portion as a heat-conduction medium.

In the hollow valve for the internal combustion engine according to the first aspect of the invention, heat of the wall portion is taken or absorbed by the high-thermal-conductivity member(s), so that the wall portion is cooled. The heat taken by the high-thermal-conductivity member(s) is transferred to a heat-conduction medium, such as a coolant, and is finally transferred from the heat-conduction medium to a cylinder head. Thus, the hollow engine valve permits highly efficient cooling of the umbrella portion that is most likely to be heated to a high temperature.

The above-indicated at least one high-thermal-conductivity member may have a directional characteristic in heat conduction between one end thereof that is in contact with the cavity-side wall and the other end that is in contact with the heat-conduction medium. For example, the high-thermal-conductivity member of this type may be formed of a carbon-fiber reinforced metal.

In the above case, the heat of the wall portion can be surely transferred to a desired heat-conduction medium.

The above-indicated at least one high-thermal-conductivity member may include a plurality of high-thermal-conductivity members that form a convex portion that protrudes in substantially the same direction as an axial direction of the stem portion, so as to transfer heat of the wall portion to the coolant.

With the above arrangement, the coolant the level of which is raised by the convex portion under the reciprocating motion of the valve reaches the upper side of the stem portion, so that the heat which the coolant takes from the high-thermal-conductivity members is transferred to the upper side of the stem portion. Thus, the hollow engine valve provides a high cooling effect, in particular, a high effect of cooling the umbrella portion.

The above-indicated at least one high-thermal-conductivity member may be placed in position so as to transfer heat between the cavity-side wall and a wall close to one end of the stem portion remote from the umbrella portion.

With the above arrangement the heat of the wall portion is transferred, without fail, directly to the wall closed to the end of the stem portion remote from the umbrella portion, so that the wall portion can be surely cooled, irrespective of whether the hollow valve itself is mounted in the engine with its axis inclined relative to the vertical direction, or even if the valve lift is small and the coolant does not rise to a high level.

The above-indicated at least one high-thermal-conductivity member may include a first high-thermal-conductivity member placed in position so as to transfer heat between a central portion of the cavity-side wall and the heat-conduction medium, and a second high-thermal-conductivity member placed in position so as to transfer heat between a peripheral portion of the cavity-side wall and a wall of the cavity which is close to a valve seat of the umbrella portion.

With the above arrangement, the heat of the wall portion is transferred to a portion of the umbrella portion that is close to the valve seat, as well as the heat-conduction medium (the coolant or stem portion). Accordingly, when it is not desired to raise the temperature of the stem portion, which would result in an increase in the temperature of intake air, the high-thermal-conductivity member are placed in position so that a relatively large amount of heat is dissipated to the portion close to the valve seat, thereby to suppress or prevent a temperature rise of the intake air.

In the hollow valve for the internal combustion engine according to the invention, the high-thermal-conductivity members have a high capability of dissipating heat, and heat is transferred from the high-thermal-conductivity members to the heat-conduction medium, so that the umbrella portion, in particular, can be favorably cooled.

An internal combustion engine according to a second aspect of the invention includes an intake valve having a structure of the hollow valve for the internal combustion engine according, to the first aspect of the invention, and an exhaust valve having a structure of the hollow valve for the internal combustion engine according to the first aspect of the invention.

In the internal combustion engine according to the second aspect of the invention, the umbrella portions of the intake valve and exhaust valve are cooled with improved efficiency, so that the engine is less likely to suffer from knocking. Owing to the effect of suppressing or preventing knocking, the ignition timing can be advanced as needed, thus assuring improved output performance of the engine.

The number of the high-thermal-conductivity members provided in the exhaust valve may be made larger than the number of the high-thermal-conductivity members provided in the intake valve.

In the above case, the larger number of high-thermal-conductivity members are provided in the exhaust valve on the exhaust side where the valve is more likely to be exposed to hot gas, than that provided in the intake valve on the air intake or induction side, thus assuring a sufficient cooling effect. Since the intake/exhaust valves used in the engine according to the second aspect of the invention can change the degree of cooling simply by adjusting the number of the high-thermal-conductivity members provided in the valve, all of the components can be shared between the exhaust valve and the intake valve, and the internal combustion engine can be manufactured at reduced cost.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements, and wherein:

FIG. 1 is a cross-sectional view showing the construction of a hollow valve for an internal combustion engine according to a first embodiment of the invention;

FIG. 2 is a perspective view showing the shape and arrangement of high-thermal-conductivity members of the hollow engine valve of the first embodiment;

FIG. 3 is a cross-sectional view showing the construction of a hollow valve for an internal combustion engine according to a second embodiment of the invention;

FIG. 4 is a cross-sectional view showing the construction of a hollow valve for an internal combustion engine according to a third embodiment of the invention;

FIG. 5 is a cross-sectional view showing the construction of a hollow valve for an internal combustion engine according to a fourth embodiment of the invention;

FIG. 6 is a top view as seen from the inside of a cavity of the hollow valve of the fourth embodiment, showing the arrangement of high-thermal-conductivity members;

FIG. 7 is a cross-sectional view showing the construction of a hollow valve for an internal combustion engine according to a fifth embodiment of the invention; and

FIG. 8 is a cross-sectional view taken along line X-X in FIG. 7, showing the arrangement of high-thermal-conductivity members of the fifth embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, some embodiments of hollow valves for internal combustion engines according to the invention will be described in detail with reference to the drawings. It is to be understood that this invention is not limited to these embodiments.

A hollow valve for an internal combustion engine according to a first embodiment of the invention will be described with reference to FIG. 1 and FIG. 2.

In FIG. 1, reference numeral 10A denotes the hollow engine valve of the first embodiment. The hollow engine valve 10A of the first embodiment may be used in an intake or exhaust valve actuating mechanism (a mechanism including, for example, a valve lifter 101) of the internal combustion engine, which is not illustrated in the drawings. In operation, the hollow engine valve 10A reciprocates in the axial direction so as to communicate a combustion chamber CC of the engine and an intake port P (or the combustion chamber CC and an exhaust port P) with each other or interrupt communication between the combustion chamber CC and the intake or exhaust port P.

The hollow engine valve 10A of the first embodiment is divided roughly into a stem portion (so-called valve stem) Vs and an umbrella portion Vh provided at one end of the stem portion Vs. The hollow valve 10A having the stem and umbrella portions Vs, Vh is constituted by a cylindrical valve body 1 having opposite open ends, a first closure member (which will be called “upper cap”) 2 that closes one of the open ends of the valve body 1, and a second closure member (which will be called “lower cap”) 3 that closes the other open end of the valve body 1.

The valve body 1 has a cylindrical portion 1 a having opposite open ends, and a truncated cone portion 1 b formed generally in the shape of a truncated cone, which has an inner space that connects the upper opening with the lower opening thereof. The cylindrical portion 1 a is connected at one of the opposite open ends to the upper open end of the truncated cone portion 1 b, to form the valve body 1 as an integral body. The inner space of the truncated cone portion 1 b is shaped like a truncated cone that is substantially the same as the outer shape or outline of the truncated cone portion 1 b. Thus, the valve body 1 has a cavity 4 formed by connecting the columnar space of the cylindrical portion 1 a with the generally truncated-cone-shaped space of the truncated cone portion 1 b. Namely, the cavity 4 of the valve body 1 is formed in substantially the same shape as the outline of the valve body 1.

In the valve body 1, the cylindrical portion 1 a forms a main part of the stem portion Vs, and the upper cap 2 is provided at the remaining open end (i.e., the open end remote from the truncated cone portion 1 b) of the cylindrical portion 1 a. Namely, the stem portion Vs of the hollow engine valve 10A illustrated herein consists of the cylindrical portion 1 a of the valve body 1 and the upper cap 2. For example, the upper cap 2 is joined integrally to the cylindrical portion 1 a by welding, or the like, thereby to close the above-indicated open end of the cylindrical portion 1 a of the valve body 1.

In the valve body 1, the truncated cone portion 1 b forms a main part of the umbrella portion Vh, and the lower cap 3 is provided at the remaining open end (i.e., the open end at the bottom) of the truncated cone portion 1 b. Namely, the umbrella portion Vh of the hollow engine valve 10A illustrated herein consists of the truncated cone portion 1 b of the valve body 1 and the lower cap 3. For example, the lower cap 3 provides a wall portion having a combustion-chamber-side wall 3 a as shown in FIG. 1, which forms a part of the wall of the combustion chamber CC when the port (intake port or exhaust port) P and the combustion chamber CC are not in communication with each other, and a cavity-side wall 3 b as shown in FIG. 2, which forms a-part of the wall of the cavity 4. The combustion-chamber-side wall 3 a and the cavity-side wall 3 b face in the opposite directions, namely, provide the opposite surfaces of the lower cap 3, or wall portion. The lower cap 3 is joined integrally to the truncated cone portion 1 b by welding, or the like, with the cavity-side wall 3 b facing inwards, so that the lower cap 3 closes the above-indicated open end of the truncated cone portion 1 b of the valve body 1.

The hollow engine valve 10A formed as described above reciprocates in the axial direction of the stem portion Vs under the action of the valve actuating mechanism. For example, the hollow engine valve 10A is pushed down by the action of a cam or rocker arm (not shown) via a valve lifter 101. As a result, the inclined face of the umbrella portion Vh of the hollow engine valve 10A moves away from an interface (i.e., an annular valve seat 102 as shown in FIG. 1) between the port (intake port or exhaust port) P and the combustion chamber CC, for communication between the port P and the combustion chamber CC.

As the hollow engine valve 10A is pushed down, an elastic member (coil spring) 104 is compressed between an annular retainer 103 fixed to the upper cap 2 and a cylinder head 105 for example. Accordingly, the spring force of the elastic member 104 acts on the hollow engine valve 10A via the retainer 103, according to the operation of the cam, or the like. As a result, the hollow engine valve 10A is moved in a direction opposite to that of the push-down action, so that the inclined face of the umbrella portion Vh abuts on the valve seat 102, thereby to interrupt communication between the port P and the combustion chamber CC.

The stem portion Vs of the hollow engine valve 10A is surrounded by a cylindrical valve stem guide 106, and is smoothly guided by the valve stem guide 106 while the valve 10A is reciprocating.

Furthermore, the hollow engine valve 10A is provided with a cooling means for dissipating heat (in particular, heat of a portion, such as the lower cap 3, that is exposed to combustion gas) transferred from the combustion chamber CC.

In the first embodiment, a coolant 5, such as metallic sodium, which serves as the cooling means is contained in the cavity 4. At least during operation of the engine, the coolant 5, if it is metallic sodium, is heated to a temperature exceeding the fusing point thereof, and is brought into a liquid state. The coolant 5 is shaken in accordance with the reciprocating motion of the hollow engine valve 10A, and is thus caused to flow in the cavity 4. Then, the coolant 5 takes heat away from a wall of the cavity 4 having a temperature higher than that of the coolant 5 when it contacts the wall, and further flows in the cavity 4 each time the valve 10A repeats the reciprocating motion, so as to transfer the heat to a wall of the cavity 4 having a temperature lower than that of the coolant 5 when it contacts the wall. In the hollow engine valve 10A of this embodiment, for example, the coolant 5 receives heat of the umbrella portion Vh that is most likely to be heated to a high temperature, and moves to the stem portion Vs under the reciprocating motion of the hollow engine valve 10A, so that the heat received from the umbrella portion Vh is transferred from the coolant 5 to the stem portion Vs, to thus effect cooling of the umbrella portion Vh.

In the first embodiment, at least one high-thermal-conductivity member 6A having high thermal conductivity and a particular directional characteristic of heat conduction is provided as another cooling means on the cavity-side wall 3 b of the lower cap 3. For example, carbon-fiber reinforced metals (CFRM) may be used for this type of high-thermal-conductivity member 6A. The carbon-fiber reinforced metal uses metal as a base material, and uses carbon fibers as a reinforcing material. The metal as the base material is exposed at the opposite ends of the high-thermal-conductivity member 6A, so that the member 6A can transfer heat from the metal exposed face of one end thereof on the high-temperature side, to the metal exposed face of the other end on the low-temperature side.

In the first embodiment, a plurality of high-thermal-conductivity members Blare disposed on the circular cavity-side wall 3 b so as to cover the entire area of the wall 3 b, as shown in FIG. 1 and FIG. 2. The high-thermal-conductivity members 6A have substantially the same length and a small diameter (of, for example, about 10 μm), and are formed straight like rods. While clearances are apparently provided between the adjacent high-thermal-conductivity members 6 in FIG. 2, these clearances are illustrated for the sake of convenience. Each of the high-thermal-conductivity members 6A is bonded at one end to the cavity-side wall 3 b by a method, such as metal plating, so that the efficiency of heat transfer between the high-thermal-conductivity member 6A and the lower cap 3 is not reduced.

Thus, in the hollow engine valve 10A of the first embodiment, the high-thermal-conductivity members 6A are formed upright over the entire area of the cavity-side wall 3 b of the lower cap 3, so that the heat of the lower cap 3 is taken or absorbed by all of the high-thermal-conductivity members 6A, and the lower cap 3 can be thus cooled. Also, in the hollow engine valve 10A, the heat is transferred to the coolant 5 that is in contact with free ends (i.e., the ends remote from the cavity-side wall 3 b) of the respective high-thermal-conductivity members 6A.

In the hollow engine valve 10A, the high-thermal-conductivity members 6A raise the level of the coolant 5 as a heat-conduction medium. Thus, if the same amount of the coolant 5 as that of a hollow engine valve having no high-thermal-conductivity members 6A is contained in the cavity 4, the coolant 5 reaches the upper portion (close to the valve lifter 101) of the stem portion Vs with higher reliability, under the reciprocating motion of the hollow valve 10A, even if the hollow engine valve 10A is mounted in the engine with its axis inclined relative to the vertical direction. Therefore, most of the heat which the coolant 5 receives from the high-thermal-conductivity members 6A (namely, the heat of the lower cap 3) is transferred to the stem portion Vs (the cylindrical portion 1 a of the valve body 1 and the lower face of the upper cap 2, which provide walls of the cavity 4), and is dissipated from the stem portion Vs to the cylinder head 105 via the valve stem guide 106, valve lifter 101, cam, and other components, Thus, the hollow engine valve 10A of the first embodiment permits highly efficient cooling of the lower cap 3; therefore, temperature rises of the valve seat 102 and the valve face of the umbrella portion Vh that abuts on the valve seat 102 are effectively suppressed or prevented, and the valve seat 102 and the valve face are less likely to wear, thus assuring improved durability and improved gas tightness of the combustion chamber CC.

On the other hand, where the hollow engine valve having no high-thermal-conductivity members 6A provides a sufficient cooling effect, the high-thermal-conductivity members 6A provided in the hollow engine valve 10A of the first embodiment serve to reduce the amount of the coolant 5 contained in the cavity 4, and reduce the inertial mass of the valve 10A when reciprocating. In this case, the hollow engine valve 10A of the first embodiment can reciprocate with increased agility and good responsiveness, thus assuring improved accuracy in the valve-opening timing and valve-closing timing.

As described above, the hollow engine valve 10A of the first embodiment permits effective cooling of the umbrella portion Vh that is most likely to be heated to a high temperature, thus assuring improvements in the durability of the valve 10A itself, durability of the valve seat 102, gas-tightness of the combustion chamber CC, and the accuracy in the valve-opening timing and valve-closing timing. Therefore, the hollow engine valve 10A achieves improved accuracy of its movements in response to a command value of the excess air ratio (i.e., the air-fuel ratio, in particular, the stoichiometric air-fuel ratio) of the combustion chamber CC, and an increased pressure in the cylinder, which lead to increased engine power and reduced fuel consumption. Also, the hollow engine valve 10A of this embodiment ensures sufficient heat resistance and durability even if the valve 10A and the valve seat 102 are formed of low-cost materials having lower heat resistance and durability than those of the related art. Thus, the hollow valve 10A and its peripheral components are available at reduced cost. In addition, since the hollow engine valve 10A of this embodiment allows the umbrella portion Vh (in particular, the combustion-chamber-side wall 3 a of the lower cap 3 that forms a part of the wall of the combustion chamber CC) to be cooled by a greater degree than that of the related art, the engine is less likely to suffer from knocking, as compared with the engine of the related art. With the hollow engine valve 10A having an effect of suppressing or preventing knocking, the ignition timing can be advanced as needed, thus assuring improved output performance.

In the hollow engine valve 10A of the first embodiment, the degree of cooling of the umbrella portion Vh can be adjusted as desired by increasing or reducing the number of the high-thermal-conductivity members 6A provided in the valve 10A. Accordingly, when the hollow engine valve 10A is used at the exhaust side (as an exhaust valve) that is more likely to be exposed to a high temperature, the larger number of the high-thermal-conductivity members 6A than that in the case where the valve 10A is used at the intake side (as an intake valve) may be provided in the valve 10A, so as to ensure a sufficient cooling effect. Since the hollow engine valve 10A of the first embodiment can change the degree of cooling simply by adjusting the number of the high-thermal-conductivity members 6A, as described above, all of the components can be shared between the exhaust side and the intake side, and exhaust valves and intake valves can be manufactured at reduced cost.

Next, a hollow valve for an internal combustion engine according to a second embodiment of the invention will be described with reference to FIG. 3. FIG. 3 is a cross-sectional view taken along the center axis of the valve body 1, and the hollow engine valve assumes the same shape or configuration as that shown in FIG. 3, when viewed in each section cut along the center axis at any angle across 360 degrees.

In FIG. 3, reference numeral 10B denotes the hollow engine valve of the second embodiment. The hollow engine valve 10B of the second embodiment is provided by replacing the high-thermal-conductivity members 6A used in the hollow engine valve 10A of the above-described first embodiment, with a plurality of high-thermal-conductivity members 6B as shown in FIG. 3.

More specifically, the high-thermal-conductivity members 6B of the second embodiment are different from the high-thermal-conductivity members 6A of the first embodiment in that the high-thermal-conductivity members 6B on a central portion of the cavity-side wall 3 b of the lower cap 3 protrude inwardly of the cavity 4. In the second embodiment, for example, a plurality of rod-like, high-thermal-conductivity members 6B having substantially the same length and diameter as those of the first embodiment are placed on an annular, peripheral portion of the cavity-side wall 3 b of the lower cap 3, and a plurality of high-thermal-conductivity members 6B that are longer than those in the peripheral portion are placed on a radially inner portion of the cavity-side wall 3 b, so that a convex portion 7 is formed inside the peripheral portion.

In the convex portion 7 of the second embodiment, the high-thermal-conductivity members 6B are formed with a length that gradually increases from the periphery of the convex portion 7, so that the height of the high-thermal-conductivity members 6B gradually increases toward the center axis of the hollow engine valve 10B.

Thus, in the hollow engine valve 10B the second embodiment, a cluster of high-thermal-conductivity members 6B on the central portion of the cavity-side wall 3 b is shaped like a mound or mountain, in other words, the high-thermal-conductivity members 6B on the central portion are formed with lengths greater than that of the members 6B on the peripheral portion so as to conform to the space shaped like a truncated cone. With this arrangement, heat is more likely to be transferred from the lower cap 3 to the high-thermal-conductivity members 6B, particularly in the convex portion 7, as compared with the first embodiment. Also, during reciprocating movements of the hollow engine valve 10B, the level of the coolant 5 as a heat-conduction medium that is in contact with the convex portion 7 can be raised to a higher level than that of the first embodiment, so that the coolant 5 reaches the upper portion (close to the valve lifter 101) of the stem portion Vs with increased reliability. Thus, the hollow engine valve 10B of this embodiment exhibits a higher cooling effect than the hollow engine valve 10A of the first embodiment.

In the case where the hollow engine valve 10A of the first embodiment provides a sufficient cooling effect, the provision of the convex portion 7 as in the second embodiment permits reduction of the amount of the coolant 5 contained in the cavity 4. In this case, the inertial mass of the hollow valve 10B when reciprocating can be reduced to be smaller than that of the first embodiment.

As described above, the hollow engine valve 10B of the second embodiment permits effective cooling of the umbrella portion Vh that is most likely to be heated to a high temperature, thus assuring further improvements in the durability of the valve 10B itself, durability of the valve seat 102, gas-tightness of the combustion chamber CC, and the accuracy in the valve-opening timing and valve-closing timing. Therefore, the hollow engine valve 10B achieves further improved accuracy of its movements in response to a command value of the excess air ratio (i.e., the air-fuel ratio, in particular, the stoichiometric air-fuel ratio) of the combustion chamber CC, and a further increased pressure in the cylinder, to more effectively increase the engine power and reduce the fuel consumption. Also, like the hollow engine valve 10A of the first embodiment, the hollow engine valve 10B of this embodiment ensures sufficient heat resistance and durability even if the valve 10B and the valve seat 102 are formed of low-cost materials having lower heat resistance and durability than those of the related art. Thus, the hollow valve 10B and its peripheral components are available at reduced cost. In addition, since the hollow engine valve 10B of this embodiment allows the umbrella portion Vh (in particular, the combustion-chamber-side wall 3 a of the lower cap 3 that forms a part of the wall of the combustion chamber CC) to be cooled by a greater degree than that of the related art, as in the first embodiment, the engine is less likely to suffer from knocking, as compared with the related art. With the hollow engine valve 10B having an effect of suppressing or preventing knocking, the ignition timing can be advanced as needed, thus assuring improved output performance.

In the hollow engine valve 10B of the second embodiment, the degree of cooling of the umbrella portion Vh can be adjusted as desired by increasing or reducing the number of the high-thermal-conductivity members 6B provided in the valve 10B, as in the first embodiment. Accordingly, when the hollow engine valve 10B is used at the exhaust side (as an exhaust valve) that is more likely to be exposed to a high temperature, the larger number of the high-thermal-conductivity members 6A than that in the case where the valve 10B is used at the intake side (as an intake valve) may be provided in the valve 10B, so as to ensure a sufficient cooling effect. Since the hollow engine valve 10B of the second embodiment can change the degree of cooling simply by adjusting the number of the high-thermal-conductivity members 6B, as described above, all of the components can be shared between the exhaust side and the intake side, and exhaust valves and intake valves can be manufactured at reduced cost.

If the hollow engine valve 10B of the second embodiment can provide a sufficient cooling effect, the high-thermal-conductivity members 6B disposed on the peripheral portion of the cavity-side wall 3 b may be removed, and the high-thermal-conductivity members 6B may be disposed only on the central portion of the wall 3 b (or in the convex portion 7), to provide a similar cooling effect. In this case, the hollow engine valve 10B in which the high-thermal-conductivity members 6B are placed over the entire area of the cavity-side wall 3 b, which provides a relatively high effect of cooling the umbrella portion Vh, may be manufactured as an exhaust valve, and the hollow engine valve 10B in which the high-thermal-conductivity members 6B are placed only on the central portion, which provides a relatively low effect of cooling the umbrella portion Vh, may be manufactured as an intake valve.

Next, a hollow valve for an internal combustion engine according to a third embodiment of the invention will be described with reference to FIG. 4. FIG. 4 is a cross-sectional view taken along the center axis of the valve body 1, and the hollow engine valve assumes the same shape or configuration as that shown in FIG. 4, when viewed in each section cut along the center axis at any angle across 360 degrees.

In FIG. 4, reference numeral 10C denotes the hollow engine valve of the third embodiment. The hollow engine valve 10C of the third embodiment is provided by replacing the high-thermal-conductivity members 6A used in the hollow engine valve 10A of the above-described first embodiment, with a plurality of high-thermal-conductivity members 6C as shown in FIG. 4.

More specifically, the high-thermal-conductivity members 6C of the third embodiment are different from the high-thermal-conductivity members 6A of the first embodiment in that the high-thermal-conductivity members 6C disposed on a central portion of the cavity-side wall 3 b of the lower cap 3 are extended up to the upper cap 2. In other words, the high-thermal-conductivity members 6B disposed in a central portion of the convex portion 7 in the second embodiment are extended up to the upper cap 2, to provide the high-thermal-conductivity members 6C of the third embodiment. In the third embodiment, for example, a plurality of rod-like high-thermal-conductivity members 6C having substantially the same length and diameter as those of the first embodiment are placed on an annular, peripheral portion of the cavity-side wall 3 b of the lower cap 3, and a plurality of high-thermal-conductivity members 6C each having one end that contacts the lower face of the upper cap 2 (namely, the face that forms a part of the wall of the cavity 4) are placed on a radially inner portion of the cavity-side wall 3 b that is located inside the peripheral portion. The high-thermal-conductivity members 6C are bonded to the upper cap 2 by a method, such as metal plating, as is the case with bonding between the members 6C and the lower cap 3.

Thus, in the hollow engine valve 10C of the third embodiment, the cavity-side wall 3 b of the lower cap 3 and the lower face of the upper cap 2 are connected to the opposite ends of the high-thermal-conductivity members 6C on the central portion, so that heat taken from the lower cap 3 can be surely transferred directly to the lower face of the upper cap 2 as a heat-conduction medium, and the heat thus transferred can be dissipated to the cylinder head 105 via the valve lifter 101, valve stem guide 106 and other components. Also, in the hollow engine valve 10C, the high-thermal-conductivity members 6C disposed on the central portion of the cavity-side wall 3 b have an increased length, and thus provide a high heat-dissipating effect. In the hollow engine valve 10C illustrated herein, the coolant 5 as a heat-conduction medium to which heat is transferred from the high-thermal-conductivity members 6C on the peripheral portion of the cavity-side wall 3 b reaches the stem portion Vs under the reciprocating motion of the valve 10C. Thus, in the hollow engine valve 10C of the third embodiment, the heat of the lower cap 3 is transferred to the upper portion of the stem portion Vs (i.e., the lower face of the upper cap 2) without fail, to surely effect cooling of the lower cap 3, irrespective of whether the valve 10 c is mounted in the engine with its axis inclined, or even if the level of the coolant 5 is not raised largely because of a small valve lift.

As described above, the hollow engine valve 10C of the third embodiment permits effective cooling of the umbrella portion Vh that is most likely to be heated to a high temperature, thus assuring further improvements in the durability of the valve 10C itself, durability of the valve seat 102, gas-tightness of the combustion chamber CC, and the accuracy in the valve-opening timing and valve-closing timing. Therefore, the hollow engine valve 10C achieves further improved accuracy of its movements in response to a command value of the excess air ratio (i.e., the air-fuel ratio, in particular, the stoichiometric air-fuel ratio) of the combustion chamber CC, and a further increased pressure in the cylinder, to more effectively increase the engine power and reduce the fuel consumption. Also, like the hollow engine valve 10A of the first embodiment, the hollow engine valve 10C of this embodiment ensures sufficient heat resistance and durability even if the valve 10C and the valve seat 102 are formed of low-cost materials having lower heat resistance and durability than those of the related art. Thus, the hollow engine valve 10C and its peripheral components are available at reduced cost. In addition, since the hollow engine valve 10C of this embodiment allows the umbrella portion Vh (in particular, the combustion-chamber-side wall 3 a of the lower cap 3 that forms a part of the wall of the combustion chamber CC) to be cooled by a greater degree than that of the related art, as in the first embodiment, the engine is less likely to suffer from knocking, as compared with the engine of the related art. With the hollow engine valve 10C having an effect of suppressing or preventing knocking, the ignition timing can be advanced as needed, thus assuring improved output performance.

In the hollow engine valve 10C of the third embodiment, the degree of cooling of the umbrella portion Vh can be adjusted as desired by increasing or reducing the number of the high-thermal-conductivity members 6C provided in the valve 10C, as is the case with the first embodiment. Accordingly, when the hollow engine valve 10C is used at the exhaust side (as an exhaust valve) that is more likely to be exposed to a high temperature, the larger number of the high-thermal-conductivity members 6C than that in the case where the valve 10C is used at the intake side (as an intake valve) may be provided in the valve 10C, so as to ensure a sufficient cooling effect. Since the hollow engine valve 10C of the third embodiment can change the degree of cooling simply by adjusting the number of the high-thermal-conductivity members 6C, as described above, all of the components can be shared between the exhaust side and the intake side, and exhaust valves and intake valves can be manufactured at reduced cost.

If the hollow engine valve 10C of the third embodiment as described above can provide a sufficient cooling effect, the high-thermal-conductivity members 6C disposed on the peripheral portion of the cavity-side wall 3 b may be removed, and the high-thermal-conductivity members 6C (that connect the cavity-side wall 3 b of the lower cap 3 with the lower face of the upper cap 2) may be disposed only on the central portion of the wall 3 b, to provide a similar cooling effect. In this case, the coolant 5 is not necessarily contained in the cavity 4 provided that a desired cooling effect can be obtained. If the coolant 5 is not contained, the inertial mass is further reduced, and the hollow engine valve 10C can reciprocate with increased agility and good responsiveness. In this case, for example, the hollow valve 10C in which the high-thermal-conductivity members 6C are placed over the entire area of the cavity-side wall 3 b, which provides a relatively high effect of cooling the umbrella portion Vh, may be manufactured as an exhaust valve, and the hollow valve 10C in which the high-thermal-conductivity members 6C are placed only on the central portion, which provides a relatively low effect of cooling the umbrella portion Vh, may be manufactured as an intake valve. Also, the hollow valve 10C in which the coolant 5 is contained may be manufactured as an exhaust valve, and the hollow valve 10C having no coolant 5 may be manufactured as an intake valve.

Next, a hollow valve for an internal combustion engine according to a fourth embodiment of the invention will be described with reference to FIG. 5 and FIG. 6.

In FIG. 5, reference numeral 10D denotes the hollow engine valve of the fourth embodiment. The hollow engine valve 10D of the fourth embodiment is provided by replacing the high-thermal-conductivity members 6A used in the hollow engine valve 10A of the above-described first embodiment with a plurality of high-thermal-conductivity members 6D₁, 6D₂ as shown in FIG. 5 and FIG. 6.

More specifically, in the fourth embodiment, the high-thermal-conductivity members 6A of the first embodiment disposed on a peripheral portion of the cavity-side wall 3 b are modified such that the free ends of the members 6A are in contact with a wall of the cavity 4 that is close to the valve face of the umbrella portion Vh. Namely, in the fourth embodiment, a plurality of rod-like, high-thermal-conductivity members 6D₁ having substantially the same length and diameter as those of the first embodiment are placed on a circular, central portion of the cavity-side wall 3 b of the lower cap 3, and a plurality of high-thermal-conductivity members 6D₂ are placed on a peripheral portion of the cavity-side wall 3 b, to extend in radial directions, such that the high-thermal-conductivity members 6D₂ connect the cavity-side wall 3 b with the wall close to the valve face at the opposite ends thereof, as shown in FIG. 6. While clearances are apparently provided between the adjacent high-thermal-conductivity members 6D₁, 6D₂ in FIG. 6, these clearances are illustrated for the sake of convenience. The high-thermal-conductivity members 6D₂ are bonded to the wall close to the valve face by a suitable method, such as metal plating, that is also used for bonding the members 6D₂ to the lower cap 3.

In the hollow engine valve 10D of the fourth embodiment, the peripheral portion of the cavity-side wall 3 b of the lower cap 3 and the wall close to the valve face are connected to each other at the opposite ends of the high-thermal-conductivity members 6D₂. With this arrangement, heat taken from the lower cap 3 can be dissipated from the wall close to the valve face as a heat-conduction medium, to the cylinder head 105, via the valve seat 102. In the hollow engine valve 10D illustrated herein, the coolant 5 as a heat-conduction medium to which heat is transferred from the high-thermal-conductivity members 6D₁ disposed on the central portion of the cavity-side wall 3 b reaches the upper portion of the stem portion Vs under the reciprocating movements of the valve 10D. Namely, in the hollow engine valve 10D of the fourth embodiment, the heat of the lower cap 3 is separately directed to the stem portion Vs and to the valve seat 102, for dissipation. With this arrangement, the hollow engine valve 10D of the fourth embodiment can surely effect cooling of the lower cap 3, as in the first embodiment.

In the case where the hollow engine valve 10D is used at the exhaust side (as an exhaust valve), the valve 10D needs to provide a greater cooling effect than that in the case where the valve 10D is used at the intake side (as an intake valve) since the whole valve is sometimes exposed to high-temperature exhaust gas, for example, on the exhaust stroke. In the case where the hollow engine valve 10D is used at the intake side (as an intake valve), on the other hand, if a large amount of heat is dissipated to the stem portion Vs that is constantly in contact with intake air, the temperature of the intake air is increased, and the volumetric efficiency in the combustion chamber CC deteriorates, which may result in a reduction of the thermal efficiency.

Accordingly, when the hollow engine valve 10D is used at the exhaust side where a high cooling capability is required of the valve 10D, the number of the high-thermal-conductivity members 6D₂ disposed on the peripheral portion and the number of the high-thermal-conductivity members 6D₁ disposed on the central portion are determined so that the larger amount of heat is dissipated to the wall close to the valve face, rather than to the stem portion Vs. Namely, since the valve face directly contacts the valve seat 102 fitted in the cylinder head 105 for conduction of heat, the heat is transferred with higher efficiency to the cylinder head 105 if the heat is dissipated to the wall close to the valve face, rather than to the stem portion Vs. Thus, the cooling capability of the hollow engine valve 10D is enhanced by increasing the amount of heat dissipated to the wall close to the valve face.

On the other hand, when the hollow engine valve 10D is used at the intake side where a cooling capability suitably controlled not to deteriorate the volumetric efficiency is required of the valve 10D, the number of the high-thermal-conductivity members 6D₂ disposed on the peripheral portion and the number of the high-thermal-conductivity members 6D₁ disposed on the central portion are determined so that the amount of heat dissipated to the stem portion Vs is reduced. In this manner, it is possible to avoid a situation where the intake air is warmed by the heat of the stem portion Vs, resulting in deterioration of the volumetric efficiency. Thus, the hollow engine valve 10D can prevent a reduction of the thermal efficiency of the engine while avoiding wasteful, excessive cooling.

Thus, the hollow engine valve 10D of the fourth embodiment can change the degree of cooling simply by adjusting the numbers of the high-thermal-conductivity members 6D₁, 6D₂ provided in the valve 10D. Therefore, all of the components can be shared between the exhaust side and the intake side, and exhaust valves and intake valves can be manufactured at reduced cost.

Also, the high-thermal-conductivity members 6D₂ of the fourth embodiment can almost uniformly distribute heat in the vicinity of a welded portion 8 as shown in FIG. 5 at which the valve body 1 and the lower cap 3 are welded to each other, and thus serve as a means for reinforcing the welded portion 8. Furthermore, the high-thermal-conductivity members 6D₂ are arranged to cover the welded portion 8, and, for this reason, too, serve as a means for reinforcing the welded portion 8. Namely, since the carbon-fiber reinforced metal of which the high-thermal-conductivity members 6D₂ are formed possesses high stiffness, the members 6D₂ can suppress or prevent warpage and distortion of the welded portion 8. Thus, the hollow engine valve 10D of the fourth embodiment can prevent cracks, or the like, from being formed in the welded portion 8 and its neighborhood.

The high-thermal-conductivity members 6D₁ disposed on the central portion may be replaced with the high-thermal-conductivity members 6B that provide the convex portion 7 in the above-described second embodiment, or may be replaced with the high-thermal-conductivity members 6C that connect the cavity-side wall 3 b of the lower cap 3 with the lower face of the upper cap 2 in the above-described third embodiment.

Next, a hollow valve for an internal combustion engine according to a fifth embodiment of the invention will be described with reference to FIG. 7 and FIG. 8.

In FIG. 7, reference numeral 10E denotes the hollow engine valve of the fifth embodiment. The hollow engine valve 10E of the fifth embodiment is provided by replacing the high-thermal-conductivity members 6A used in the hollow engine valve 10A of the above-described first embodiment, with a plurality of high-thermal-conductivity members 6E as shown in FIG. 7 and FIG. 8.

More specifically, each of the high-thermal-conductivity members 6E of the fifth embodiment connects a peripheral portion of the cavity-side wall 3 b of the lower cap 3 with a wall of the cavity 4 that is close to the valve stem guide 106 at the opposite ends thereof. The high-thermal-conductivity members 6E are arranged in radial directions as shown in FIG. 8, to radiate out from the center axis of the hollow engine valve 10E. While clearances are apparently provided between the adjacent high-thermal-conductivity members 6E in FIG. 8, these clearances are illustrated for the sake of convenience. The high-thermal-conductivity members 6E are bonded to the wall close to the valve stem guide 106 by a suitable method, such as metal plating, that is also used for bonding the members GE to the lower cap 3.

Thus, in the hollow engine valve 10E of the fifth embodiment, the peripheral portion of the cavity-side wall 3 b of the lower cap 3 and the wall close to the valve stem guide 106 are connected to the opposite ends of the high-thermal-conductivity members 6E, to be thus connected to each other. With this arrangement, heat taken from the lower cap 3 can be surely transferred directly to the wall close to the valve stem guide 106 as a heat-conduction medium, and the heat thus transferred can be dissipated to the cylinder head 105 via the valve stem guide 106. Also, the high-thermal-conductivity members 6E of the hollow engine valve 10E have a relatively long length, and thus provide a high heat-dissipating effect. In the hollow engine valve 10E illustrated herein, the coolant 5 that takes heat from the central portion of the cavity-side wall 3 b reaches the stem portion Vs under the reciprocating movements of the valve 10E. Accordingly, in the hollow engine valve 10E of the fifth embodiment, the heat of the lower cap 3 is transferred to the wall close to the valve stem guide 106 without fail, irrespective of whether the valve 10E is mounted in the engine with its axis inclined, or even if the level of the coolant 5 is not raised to a sufficiently high level because of a small valve lift. Thus, the lower cap 3 is cooled with high reliability.

While a large number of high-thermal-conductivity members are arranged at a high density in the first through fourth embodiments, a relatively small number of high-thermal-conductivity members 6E are provided in the fifth embodiment; therefore, the high-thermal-conductivity members 6E may be formed with a relatively large cross-sectional area (i.e., the cross-sectional area of metal as a base material), which leads to an increase in the amount of heat dissipated from the members 6E.

As described above, the hollow engine valve according to the present invention is useful or advantageous in terms of the cooling capability, in particular, the capability of cooling the umbrella portion.

While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention. 

1. A hollow valve for an internal combustion engine, comprising: a stem portion; an umbrella portion provided at one end of the stem portion, said stem portion and said umbrella portion cooperating with each other to form a gastight cavity that communicates an interior of the stem portion with an interior of the umbrella portion; and a coolant contained in the cavity, wherein: the umbrella portion has a wall portion having a combustion-chamber-side wall that forms a part of a wall of a combustion chamber of the internal combustion engine, and a cavity-side wall that forms a part of a wall of the cavity; and at least one high-thermal-conductivity member is provided on the cavity-side wall, for conduction of heat between the wall portion of the umbrella portion and at least one of the coolant and the stem portion as a heat-conduction medium.
 2. The hollow valve according to claim 1, wherein said at least one high-thermal-conductivity member has a directional characteristic in heat conduction between one end thereof that is in contact with the cavity-side wall and the other end that is in contact with the heat-conduction medium.
 3. The hollow valve according to claim 1, wherein said at least one high-thermal-conductivity member comprises a carbon-fiber reinforced metal.
 4. The hollow valve according to claim 3, wherein: the carbon-fiber reinforced metal includes a metal, and carbon fibers; and the metal is exposed at the opposite ends of the carbon-fiber reinforced metal.
 5. The hollow valve according to claim 1, wherein said at least one high-thermal-conductivity member comprises a plurality of high-thermal-conductivity members that form a convex portion that protrudes in substantially the same direction as an axial direction of the stem portion, so as to transfer heat of the wall portion to the coolant.
 6. The hollow valve according to claim 5, wherein said plurality of high-thermal-conductivity members that form the convex portion are formed such that the length of the high-thermal-conductivity members increases toward a central portion of the cavity-side wall.
 7. The hollow valve according to claim 1, wherein said at least one high-thermal-conductivity member is placed in position so as to transfer heat between the cavity-side wall and a wall close to one end of the stem portion remote from the umbrella portion.
 8. The hollow valve according to claim 1, wherein said at least one high-thermal-conductivity member comprises a first high-thermal-conductivity member placed in position so as to transfer heat between a central portion of the cavity-side wall and the heat-conduction medium, and a second high-thermal-conductivity member placed in position so as to transfer heat between a peripheral portion of the cavity-side wall and a wall of the cavity which is close to a valve seat of the umbrella portion.
 9. The hollow valve according to claim 1, wherein said at least one high-thermal-conductivity member comprises a plurality of high-thermal-conductivity members each connecting a peripheral portion of the cavity-side wall with a wall of the cavity which is close to a valve stem guide that surrounds a part of the stem portion, at the opposite ends thereof, said plurality of high-thermal-conductivity members being arranged in radial directions to radiate from a center axis of the hollow valve.
 10. An internal combustion engine, comprising: an intake valve having a structure of the hollow valve for the internal combustion engine as defined in claim 1; and an exhaust valve having a structure of the hollow valve for the internal combustion engine as defined in claim
 1. 11. The internal combustion engine according to claim 10, wherein the number of the high-thermal-conductivity members provided in the exhaust valve is larger than the number of the high-thermal-conductivity members provided in the intake valve. 